docs: def get_elastix_exes():
      """ Get the executables for elastix and transformix. Raises an error
      if they cannot be found.
      """
      if EXES:
          if EXES[0]:
              return EXES
          else:
              raise RuntimeError('No Elastix executable.')
      
      # Find exe
      elastix, ver = _find_executables('elastix')
      if elastix:
          base, ext = os.path.splitext(elastix)
          base = os.path.dirname(base)
          transformix = os.path.join(base, 'transformix' + ext)
          EXES.extend([elastix, transformix])
          print('Found %s in %r' % (ver, elastix))
          return EXES
      else:
          raise RuntimeError('Could not find Elastix executable. Download '
                             'Elastix from http://elastix.isi.uu.nl/. Pyelastix '
                             'looks for the exe in a series of common locations. '
                             'Set ELASTIX_PATH if necessary.')